Benefit Plans (Funded Status of Benefits Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Change in benefit obligation:
Net benefit obligation at January 1	$ 1,244.3	$ 1,206.3
Service cost	12.5	12.6	$ 15.9
Interest cost	51.0	53.0	53.6
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	83.6	48.3
Gross benefits paid	(88.3)	(75.9)
Net benefit obligation at December 31	1,303.1	1,244.3	1,206.3
Change in plan assets:
Fair value of plan assets at January 1	895.7	895.0
Actual return on plan assets	136.7	74.3
Employer contributions	6.6	2.3
Plan participants' contributions	0.0	0.0
Gross benefits paid	(88.3)	(75.9)
Fair value of plan assets at December 31	950.7	895.7	895.0
Under funded status at December 31	(352.4)	(348.6)
Pension Plans, Defined Benefit [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	570.4	556.1
Service cost	7.3	7.5	8.8
Interest cost	23.5	24.5	25.0
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	34.9	19.1
Gross benefits paid	(43.2)	(36.8)
Net benefit obligation at December 31	592.9	570.4	556.1
Change in plan assets:
Fair value of plan assets at January 1	422.0	422.7
Actual return on plan assets	64.3	35.3
Employer contributions	0.6	0.8
Plan participants' contributions	0.0	0.0
Gross benefits paid	(43.2)	(36.8)
Fair value of plan assets at December 31	443.7	422.0	422.7
Under funded status at December 31	(149.2)	(148.4)
Pension Plans, Defined Benefit [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	529.2	505.9
Service cost	4.9	4.9	5.8
Interest cost	21.8	22.3	22.6
Plan participants' contributions	0.0	0.0
Actuarial (gain) loss	38.3	25.7
Gross benefits paid	(34.4)	(29.6)
Net benefit obligation at December 31	559.8	529.2	505.9
Change in plan assets:
Fair value of plan assets at January 1	389.7	386.8
Actual return on plan assets	59.6	32.4
Employer contributions	0.1	0.1
Plan participants' contributions	0.0	0.0
Gross benefits paid	(34.4)	(29.6)
Fair value of plan assets at December 31	415.0	389.7	386.8
Under funded status at December 31	(144.8)	(139.5)
Other Postretirement Benefits Plans [Member]
Change in benefit obligation:
Net benefit obligation at January 1	220.1	221.4
Service cost	5.0	5.3	5.5
Interest cost	8.6	9.4	9.1
Plan participants' contributions	2.9	2.4
Actuarial (gain) loss	5.4	(0.3)
Gross benefits paid	(19.7)	(18.1)
Net benefit obligation at December 31	222.3	220.1	221.4
Change in plan assets:
Fair value of plan assets at January 1	105.8	106.9
Actual return on plan assets	12.9	8.2
Employer contributions	9.2	6.4
Plan participants' contributions	2.9	2.4
Gross benefits paid	(19.7)	(18.1)
Fair value of plan assets at December 31	111.1	105.8	106.9
Under funded status at December 31	(111.2)	(114.3)
Other Postretirement Benefits Plans [Member] | IPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	90.1	91.3
Service cost	2.1	2.3	2.4
Interest cost	3.5	3.8	3.8
Plan participants' contributions	1.0	0.9
Actuarial (gain) loss	(0.1)	(0.7)
Gross benefits paid	(7.2)	(7.5)
Net benefit obligation at December 31	89.4	90.1	91.3
Change in plan assets:
Fair value of plan assets at January 1	68.2	69.2
Actual return on plan assets	8.9	5.3
Employer contributions	2.0	0.3
Plan participants' contributions	1.0	0.9
Gross benefits paid	(7.2)	(7.5)
Fair value of plan assets at December 31	72.9	68.2	69.2
Under funded status at December 31	(16.5)	(21.9)
Other Postretirement Benefits Plans [Member] | WPL [Member]
Change in benefit obligation:
Net benefit obligation at January 1	88.9	89.7
Service cost	1.9	2.0	2.1
Interest cost	3.4	3.8	3.7
Plan participants' contributions	1.4	1.2
Actuarial (gain) loss	4.1	0.5
Gross benefits paid	(9.3)	(8.3)
Net benefit obligation at December 31	90.4	88.9	89.7
Change in plan assets:
Fair value of plan assets at January 1	18.6	18.7
Actual return on plan assets	1.2	1.2
Employer contributions	6.8	5.8
Plan participants' contributions	1.4	1.2
Gross benefits paid	(9.3)	(8.3)
Fair value of plan assets at December 31	18.7	18.6	$ 18.7
Under funded status at December 31	$ (71.7)	$ (70.3)